Alkylation of aromatics is well known in the art and is usually carried out by the reaction of an alkyl halide with an aromatic hydrocarbon in the presence of a Lewis acid catalyst such as hydrofluoric acid, boron trifluoride, concentrated sulphuric acid, zeolites and combinations thereof. More recently, acidic catalysts such as aluminium halides and the alkyl aluminium halides have been preferred, optionally in combination with a co-catalyst such as an alkyl halide. For instance, styrene has been produced by the catalytic dehydrogenation of ethyl benzene which in turn is derived by direct alkylation of benzene with ethylene in the presence of such a catalyst. Such alkylation processes are described in detail by eg Olah, G A., ed. Friedel-Crafts and Related Reactions, Interscience Publishers, J Wiley & Sons, New York (1964) and by Streitweiser, A and Reif, L., J Am Chem Soc 82, 5003 (1960). These processes use two reactors of which one is a so-called "primary alkylator" and the other is a so-called "trans- or de-alkylator". Both reactors are constructed from acid resistant materials eg Hastalloy.RTM.B or furan-lined firebrick in a 316 SS casing. Ethylene and benzene are fed into the primary alkylator in mole ratios in the range of 0.1:0.9. A catalyst complex is formulated eg by adding aluminium powder to benzene in the presence of ethyl chloride (which combination represents the promoter and catalyst) and small quantities of polyethylbenzenes. A `red oil` catalyst complex is thus generated which is then fed into the primary alkylator in an amount which is &lt;1% w/w of the total reactants in the primary alkylator. The `red oil` is fed to the primary alkylator such that it totally dissolves in the benzene/ethylene system. The resultant mixture is subjected to controlled reaction conditions such that the effluent from the primary alkylator contains a mixture of unreacted benzene, ethyl benzene (the desired product), and by-products including diethyl benzene and polyethyl benzenes. This effluent stream is combined with recycled polyethyl benzenes in the trans-alkylator. The contents of the trans-alkylator, which are liquids, reach a composition which may or may not be thermodynamically controlled depending upon catalyst activity. Invariably, the effluent from the trans-alkylator will be different in composition from the primary alkylator as it will contain less benzene and more of the desired ethyl benzene. The effluent from the trans-alkylator is washed with water and alkali, eg sodium hydroxide or ammonia to destroy the catalyst. This means that that the catalyst is irretrievably lost and cannot therefore be recycled or reused. The washed material is then distilled to produce benzene for recycle, the desired ethyl benzene, di- and tri-ethyl benzenes for recycle and the remaining heavy materials such as tetra-, penta- and hexa-ethyl benzenes and impurities are recovered and used as `flux oil`. The ethyl benzene is recovered and kept in a suitable storage facility eg in tanks, until required for the dehydrogenation stage to produce styrene.
Such catalysts have the disadvantage that some of the acids used such as hydrofluoric acid are too strong, corrosive and volatile and therefore require a considerable degree of safety measures both for the equipment used and for the operational personnel involved; others such as concentrated sulphuric acid are relatively inactive and require high reaction volumes and expensive reconcentration equipment; and zeolites are too weak therefore requiring the use of relatively high reaction temperatures. Where aluminium halides are used, they are usually miscible with the reactant hydrocarbons and have to be destroyed to recover the desired alkylated product thereby preventing recycle of the catalyst and hence adding to the cost of the process.
Alkylation of isoparaffins by olefins has also been reported in FR-A-2626572 in the presence of a catalyst comprising ionic liquids. This document, however, does not describe the alkylation of aromatics using olefins. The process of alkylation of iso-paraffins is different from that involving the alkylation of aromatics because, surprisingly, these catalysts for the alkylation of iso-paraffins--where relatively severe reaction conditions are required to obtain the desired species with high octane rating--can also catalyse the alkylation of aromatics with the desired conversion-selectivity.
Ionic liquids are primarily mixtures of salts which melt below room temperature. Such salt mixtures include aluminium halides in combination with one or more of imidazolium halides, pyridinium halides or phosphonium halides and the latter being preferably substituted. Examples of the latter include one or more of 1-methyl-3-butyl imidazolium halides, 1-butyl pyridinium halide and tetrabutyl phosphonium halides.
It has now been found that aromatic hydrocarbons can be alkylated directly with olefins without recourse of alkyl halides as the alkylating agent whilst at the same time mitigating the problems generated by the use of strong acids, expensive equipment or inability to recycle the catalysts used as described above.